During the combustion process, gasoline is oxidized, and hydrogen (H) and carbon (C) combine with air. Various chemical compounds are formed including carbon dioxide (CO2), water (H2O), carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons (HC), sulfur oxides (SOx), and other compounds.
Automobile exhaust systems include a three-way catalytic converter that helps oxidize CO, HC and reduce NOx in the exhaust gas. The efficiency of the catalytic converter is periodically monitored to prevent excess CO, HC and NOx in the exhaust gas. Typically, the catalytic converter is monitored during engine steady state operating conditions. At idle, for example, the engine controller adjusts the air to fuel (A/F) ratio to achieve consistent emissions output. Traditional monitoring methods force the A/F ratio to a lean or rich condition for a predetermined period. Afterwards, the controller switches to the rich or lean condition. The controller estimates an oxygen storage capacity (OSC) of the catalytic converter based on exhaust data (e.g., the lag time between an inlet oxygen sensor and an outlet oxygen sensor detecting the lean/rich condition). The OSC is indicative of the efficiency of the catalytic converter.
Existing monitoring methods do not accurately compensate OSC measurements over catalyst temperature and mass air flow ranges. As a result, there is a lower separation between good and bad converters and increases the number of false FAIL and/or false PASS decisions.